This invention relates to reflect array antennas, and more particularly to a microstrip asymmetric-element phase shifting reflect array antenna.
Many radar, electronic warfare and communication systems require a circularly polarized antenna with high gain and low axial ratio. Conventional mechanically scanned reflector antennas are available to meet these specifications. However, such antennas are bulky, difficult to install, and subject to performance degradation in winds. Planar phased arrays may also be employed in these applications. However, these antennas are costly because of the large number of expensive GaAs Monolithic microwave integrated circuit components, including an amplifier and phase shifter at each array element as well as a feed manifold and complex packaging. Furthermore, attempts to feed each microstrip element from a common input/output port becomes impractical due to the high losses incurred in the long microstrip transmission lines, especially in large arrays.
Conventional microstrip reflect array antennas use an array of microstrip antennas as collecting and radiating elements. Conventional reflect array antennas use either delay lines of fixed lengths connected to each microstrip element to produce a fixed beam or use an electronic phase shifter connected to each microstrip element to produce an electronically scanning beam. These conventional reflect array antennas are not desirable because the fixed beam reflect arrays suffer from gain ripple over the reflect array operating bandwidth, and the electronically scanned reflect array suffer from high cost and high phase shifter losses.
It is also known that a desired phase variation across a circularly polarized array is achievable by mechanically rotating the individual circularly polarized array elements. Miniature mechanical motors or rotators have been used to rotate each array element to the appropriate angular orientation. However, the use of such mechanical rotation devices and the controllers introduce mechanical reliability problems. Further, the manufacturing process of such antennas are labor intensive and costly.
In U.S. Pat. No. 4,053,895 entitled xe2x80x9cElectronically Scanned Microstrip Antenna Arrayxe2x80x9d issued to Malagisi on Oct. 11, 1977, antennas having at least two pairs of diametrically opposed short circuit shunt switches placed at different angles around the periphery of a microstrip disk is described. The shunt switches connect the periphery of the microstrip disk to a ground reference plane. Phase shifting of the circularly polarized reflect array elements is achieved by varying the angular position of the short-circuit plane created by diametrically opposed pairs of diode shunt switches. This antenna is of limited utility because of the complicated labor intensive manufacturing process required to connect the shunt switches and associated bias network between the microstrip disk and ground, as well as the cost of the circuitry required to control the diodes.
In accordance with the present invention, there is provided a reflect array antenna providing electronic beam scanning at low cost. The reflect array antenna of the present invention enables an increase in the number of phase states for the reflect array elements, while reducing the number of switches required to provide electronic beam scanning. The reflect array antenna of the present invention provides increased performance for a given frequency, that is, a greater number of discreet phase states for a given number of switches. Alternatively, the described reflect array antenna provides improved performance (number of phase states) at a higher frequency due to the ability to utilize fewer switches and therefore provide phase shift integration. This enables the claimed reflect array antenna to be used as an electronically steered array (ESA) at millimeter wave frequencies for applications requiring low cost, for example, millimeter wave communication apertures, and millimeter wave missile seekers.
In accordance with the present invention, there is provided a reflect array antenna comprising a non-electrically conductive substrate with the antenna array supported on the substrate. Each array of the antenna comprises patch antenna elements having a plurality of notches formed in the antenna element, the notches are angularly displaced around the circumference of the element. A plurality of stub short transmission lines are individually positioned in each of the plurality of notches. A plurality of switches are individually coupled to an end of one notch and to one of the plurality of stub short transmission lines.
Further in accordance with the present invention, there is provided an antenna element for a reflect array antenna comprising as an element thereof a non-electrically conductive substrate. Supported on the substrate is a patch antenna element having a plurality of notches formed in the element, the notches are angularly displaced around a circumference of the element. A plurality of stub short transmission lines are individually positioned in each of the plurality of notches and a plurality of switches individually couple an end of one notch to one of the plurality of stub short transmission lines.
Further in accordance with the present invention, there is provided a circularly polarized reflect array antenna comprising a support base and plurality of antenna. subarrays mounted to the support base. Each antenna subarray comprises a non-electrically conductive substrate with a patch antenna supported on the substrate. Each patch antenna of the array comprises a patch antenna element having a plurality of notches formed in the antenna element, the notches are angularly displaced around the circumference of the element. A plurality of stub short transmission lines are individually positioned in each of the plurality of notches, and a plurality of switches are individually coupled to an end of one notch and to one of the plurality of stub short transmission lines. In addition, the circular polarized reflect array antenna comprises a feed horn coupled to the support base for transmitting or receiving radio frequency energy to a subreflector, the subreflector focusing the radio frequency energy received by the plurality of antenna subarrays to the feed horn.
A technical advantage of the present invention is a simplified method for building an electronic scanning reflect array antenna. The advantages of the present invention are achieved by an antenna containing a lattice of circular patch antennas with perimeter stubs connected to the patches by switches. A further advantage of the present invention is a reduction of the number of stub short transmission lines and switches required to control beam steering.